In butterfly valves, translational movement of the valve plate relative to the pivot mechanism can be used to effect dynamic torque balancing, and to facilitate actuation by using the torque exerted by fluid on the plate to rotate the latter. To applicant's knowledge, these principles were first set forth in the above-referenced patent application, Ser. No. 374,897, filed on Jun. 30, 1989. They are schematically illustrated in FIGS. 12A-12C of the present application wherein numerals 20, 22, 24, 26, and 28 indicate a valve plate, a pivot shaft, a duct, a bracket intersecuring the shaft and plate, and the direction of flow respectively. Arrow 30 and numeral 32 indicate the center of pressure associated with the forces exerted by fluid on the plate 20, and the pivot line about which the plate is rotatable.
In FIG. 12A, the shaft 22 is centered in relation to the longitudinal dimension of the plate 20 and the plate is in a closed position. The center of pressure 30 is aligned with the pivot line 32, so the net torque exerted by the fluid on the plate 20 is zero. If the plate 20 is rotated to an open position as illustrated in FIG. 12B, and translated relative to the shaft 22 so that the center of pressure 30 is aligned with the pivot line 32 as illustrated in FIG. 12C, then the plate is effectively torque-balanced and the flow rate associated with the rotational angle 34 can be maintained with minimal input force required from the actuator 36. If the translational movement is too limited in range to achieve torque balancing at all rotational positions of the plate 20, it can still be used to advantage in minimizing the net torque exerted by the fluid, thus still minimizing the input force required from the actuator 36.
If it is desired to change the rotational angle 34, then the plate 20 can be translated relative to the shaft 22 in the appropriate direction to effect a misalignment of the center of pressure 30 with the pivot line 32, thus increasing the net fluid dynamic torque, and the force required to effect the change is exerted in whole or in part by the fluid conveyed in the duct 24.
Butterfly valves that are designed to provide for translational movement of the plate 20 relative to the shaft 22, for either or both of the above-described purposes, are referred to herein as "balanced-torque" butterfly valves.
Upon reflection, it will be understood that the use of balanced-torque butterfly valves in applications which demand a rotational angle 34 corresponding to a closed position presents the problem of providing for translational movement while also providing for initial opening of the valve plate 20. The problem may be particularly pronounced when the application requires sealing engagement of the valve plate 20 with the inner surface of the duct 24 at the closed position. If a rim seal is employed, the frictional force between the seal and the duct increases the torque required to rotate the valve plate away from the closed position. The problem is further exacerbated if the upstream fluid pressure is considerably high. In that case, the fluid may exert radially outward pressure on the rim seal and further increase the torque required to open the valve. A key advantage associated with balanced-torque butterfly valves is that smaller actuators can be used to control the rotational position of the valve plate. However, if a single actuator is used for both initial opening and subsequent movement of the valve plate, then the forementioned advantage is unrealized to the degree that power requirements from the actuator are dictated by the torque required for initial opening.